Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus, and particularly, to a switching power supply apparatus using an active clamp system in an insulation converter using a flyback transformer.
Description of the Related Art
In a switching power supply, such as a commercial power supply, that converts an AC voltage to a DC voltage, improvement of efficiency of the switching power supply is required to reduce power consumption of the switching power supply. The efficiency of the switching power supply is expressed by a ratio of power output by the switching power supply to power supplied to the switching power supply.
In a switching power supply using an active clamp system in an insulation converter using a flyback transformer, a configuration of Japanese Patent No. 4370844 is proposed as an example of a unit that improves efficiency in a state that the switching power supply outputs low power. Hereinafter, the state that the switching power supply outputs low power will be referred to as a low load state.
However, further improvement of the efficiency in the low load state is required in the switching power supply using the active clamp system.
Two switching elements are alternately turned on and off in the switching power supply using the active clamp system in the insulation converter using the flyback transformer. Consequently, part of energy stored in the transformer is transmitted to a secondary side by flyback operation, and the rest of the energy is resonated on a primary side to switch the switching element with zero voltage. In this way, the power on the primary side can be highly efficiently converted to the secondary side by using the active clamp system. Therefore, high power conversion efficiency can be realized.
A circuit configuration is proposed for example in Japanese Patent Application Laid-Open No. H11-187664, in which turn-on times of two switching elements are appropriately controlled based on a load, and high power conversion efficiency is attained in a wide load range. A circuit configuration is proposed for example in Japanese Patent Application Laid-Open No. 2013-201829, in which turn-on times of two switching elements are changed according to an input voltage to handle fluctuations of the input voltage.
However, realization of a switching power supply that can flexibly handle specifications required for each of a plurality of target output voltages of control (hereinafter, “target voltage”) is required.